Parallelize verify_funding: Tracking libifiication via #10

[DanGould]

#10 is a big rewrite. Let's integrate it piecemeal so it gets the thorough review it deserves. The following list are the changes from that list that have made it into master.

• lnd: Contains the lnd-client codebase.
• scheduler: Contains the business logic of scheduling and "payjoin-channel-funding" requests.
• api: Contains all the endpoint logic (currently only for HTTP).

lnd:

• The lnd client (now named lnd::LndClient) now implements Sync and Send allowing for better parallelism. Split lightning client logic from main #11
• Lnd client "Verify funding" logic now interacts with remote lightning nodes in parallel (lnd::LndClient::verify_funding).
• Added lnd::LndError to properly catch errors instead of panicing. Split lightning client logic from main #11

scheduler:

• ScheduledPayJoin includes additional member functions for checkout output amounts and generating open_channel messages (from it's scheduled channels). Split Modules from main function #17
• ScheduledPayJoin::test_connections now tests connections in parallel.
• Introduce scheduler::ProposalPsbt (which needs a better name), that handles state for satisfying a scheduled payjoin-channel-funding.
• Introduce SchedulerError for handling error cases instead of panicing.
• Scheduler contains the business logic.

api:

• Handles HTTP requests with a given Scheduler and options.
• Introduce ApiError which can turn into a non-SUCCESS HTTP response instead of panicing.
• We embed static files into the binary with include_bytes! macro instead of requiring specification of static file location.

[evanlinjin]

ScheduledPayJoin is what is scheduled by the "admin/controller/owner_of_lnd_noder". Basically saying, "if we get an original psbt (bip 78), which has an output spending to this address, we should schedule these channel opens (which so happens to be formed as a payjoin tx)".

ProposalPsbt is the "state" of handling a receive of an original psbt. Badly named. I am sorry (I think I proposed a better name in the comments).